Coded systems, which combine both Orthogonal Frequency Division Multiplexing (OFDM) and channel coding techniques, are used in communication systems to efficiently transmit high rate signals in fading channels. Due to the wide bandwidth of these signals they would normally suffer from severe frequency selective fading or isolated narrowband interference.
This is avoided in an OFDM system by dividing the data into multiple parallel data streams or channels, one for each subcarrier, where the transmission in each individual subcarrier experiences only flat frequency fading. Each subcarrier is modulated with a conventional modulation scheme such as quadrature amplitude modulation (QAM) at a low symbol rate, maintaining total data rates similar to conventional single carrier modulation schemes in the same bandwidth. Forward Error Correction (FEC) coding is used to combat errors resulting from noise, faded OFDM subcarriers, or isolated narrowband interference.
One of the primary disadvantages of OFDM is that it possesses a high peak-to-average-power ratio (PAPR) that requires the use of linear power amplifiers which are less efficient than nonlinear ones. Amplifier nonlinearity exhibits amplitude and phase distortions which cause loss of orthogonality among the subcarriers and introduces inter-carrier interference in the transmitted signal which increases bit error rate (BER). A number of techniques have been postulated to reduce PAPR but all suffer from either high complexity or low PAPR reduction. Amplitude clipping is the simplest technique but increases BER.
Accordingly, it is the object of the present invention to disclose methods which provide improved COFDM wireless communication system performance by reducing required signal-to-noise-ratio (SNR) and increasing immunity to amplifier nonlinearity.